Prostate cancer is the most common cancer among men in the United States and genetic susceptibility is one of the strongest risk factors for this disease. Although the roles of tumor suppressor genes such as PTEN and CDKN1B in tumorigenesis have been well established in multiple cancers, including prostate, studies that examined the independent effect of germline mutations of PTEN and CDKN1B on prostate cancer risk have been limited and generally unsuccessful. Recently, a mouse study clearly demonstrated that combinations of genetic effects at both Pten and Cdknlb, rather than a single gene, cause prostate cancer in mice. In addition, the linkage results from our genome-wide screen also demonstrated a strong interaction between the chromosomal regions at 10q23 (PTEN) and 12pl 3 (CDKNIB). We therefore hypothesize that the interaction of germline mutations in two tumor suppressor genes (PTEN and CDKNIB) affects individual susceptibility to hereditary and non-hereditary prostate cancer. Four specific aims are proposed to test this hypothesis: 1) To sequence the entire transcript and promoter regions of CDKNIB and potentially important regions of PTEN to identify mutations and sequence variants among 188 high risk HPC probands and perform bioinformatic analysis to predict the biological significance of the identified variants; 2) To assess the main effect of each gene and the interaction effect of the two genes in hereditary prostate cancer by testing for linkage and association of the identified mutations/sequence variants with prostate cancer in all 188 HPC families using one-locus and two-locus linkage and association analyses; 3) To assess the main effect of each gene and the interaction effect of the two genes in non-hereditary prostate cancer by testing for association between the identified mutations/sequence variants and prostate cancer in a case-control population using single SNP, haplotype, and MDR approaches; and 4) To evaluate the functional impact of the mutations/sequence variants by measuring the expression of mRNA and protein levels, and alteration in protein functions, for a subset of mutations/sequence variants that are implicated in linkage and association analyses. The results from this study are likely to significant advance our knowledge of prostate cancer risk and begin to explore the underlying biological mechanisms for any observed differences.